1. Field of the Invention
The present invention relates to a gas insulated apparatus and particularly relates to a gas insulated apparatus in which a grounded closed vessel is filled with an insulating gas so that an electrically live part disposed in the vessel is insulated by the insulating gas.
2. Background of the Invention
Recently, a gas insulated switch-gear has been widely used in an electric power station in order to reduce the space for mounting electric power equipment and increase safety of the same. In this gas insulated switch gear, a circuit breaker, a disconnecting switch, a bus, etc., are disposed in a grounded metal vessel filled with an insulating gas such as an SF.sub.6 gas so as to establish insulation to the ground as well as inter-phase insulation by the insulating gas.
FIG. 3 schematically shows the arrangement of such a gas insulated switch-gear as described above. The gas insulated switch-gear is constituted by conductors 21 which are hot or electrically live parts, disconnecting switches 22 and circuit breakers 23 both connected to the conductors 21. The conductors 21 passed through current transformers 24. Metal vessels 25 enclosing the fore going components. Insulating spacers 26 partition spaces in the metal vessels 25 and support the conductors 21. Bushings 27 fixed to end portions of the respective metal vessels 25 support the conductors 21. The metal vessels 25 are filled with SF.sub.6 gas 28. In such a gas insulated switch-gear, an arc may be generated between each of the conductors 21 and the associated grounded metal vessel 25 when an abnormal voltage is applied to the conductor 21. However, the arc can be extinguished by the arc-extinction action of the SF.sub.6 gas 28 when the voltage is returned to its normal state.
In the thus arranged gas insulated switch-gear, assuming that an abnormal voltage V.sub.S enters onto the conductors 21 from a transformer 29 to thereby generate a ground fault at one point, for example, at a point A as shown in the drawing, a ground fault current I.sub.gs substantially of a value as shown by the following expression (1) flows, EQU I.sub.gs =V.sub.S /(Z.sub.fa +Z.sub.n +Z.sub.t).apprxeq.V.sub.S /(Z.sub.n +Z.sub.t) (1)
where Z.sub.fa represents an impedance at the fault point A; Z.sub.n, an impedance at a neutral point of the transformer 29; and Z.sub.t, an internal impedance of the transformer 29.
Thereafter, when the abnormal voltage V.sub.S is returned to a normal one V.sub.E, the ground fault current I.sub.ge assumes a value as expressed by the following expression (2). EQU I.sub.ge =V.sub.E /Z.sub.n ( 2)
In this case, if the neutral point impedance Z.sub.n is suitably determined so that the ground fault current I.sub.ge is smaller than a self-arc-extinction current at which an arc is self extinguished owing to the SF.sub.6 gas 28, the arc of the ground fault current I.sub.ge may be rapidly extinguished.
If a multi-phase ground fault is generated at three different phase points, for example, at the points A, B and C as shown in the drawing, however, no current flows through the neutral point impedance Z.sub.n, so that the ground fault current I.sub.ge does not decrease below the self-arc-extinction current and the earth fault current I.sub.ge is left to flow as it is.
Alternatively, if a multi-phase ground fault is generated at two different phase points, for example, the points A and B, the ground fault current I.sub.ge in this case becomes a value as expressed by the following expression (3). ##EQU1## where Z.sub.fb represents an impedance at the fault point B.
The ground fault current I.sub.ge in this case is remarkably larger than that in the case where a ground fault is generated at one point, such as the point A, as described above, because the impedance (Z.sub.fa +Z.sub.fb +Z.sub.t) in expression (3) is much smaller than the neutral point impedance Z.sub.n and a voltage of .sqroot.3V.sub.E is applied across the two faulted conductors 21. p If a ground fault is generated and a ground fault current I.sub.ge continues to flow, the danger occurs that the pressure in the metal vessel 25 is made so high owing to the arc as to thereby cause the vessel 25 to explode or the metal vessel 25 is partly melted to be broken by the fixed root of the arc. In a switch-gear, therefore, it is desired that even if a multi-phase earth fault occurs to generate an arc, the arc can be extinguished immediately as soon as the abnormal voltage V.sub.S is returned to the normal value V.sub.E.